1. Field of The Invention
The present invention relates to sockets for integrated circuit chip carriers having flexible leads and, more particularly to a low profile chip carrier socket of a surface mount type for electrically connecting such a carrier to conductors on a substrate.
2. Description of The Prior Art
With regard to chip carrier sockets, the prior art can be characterized by basically two types. The first type can be represented by Plastic Leaded Chip Carrier (PLCC) socket in which the chip carrier is retained only by means of engagement between the contacts of the socket and the leads of the chip carrier, for example, U.S. Pat. Nos. 4,630,875, 4,645,279, 4,679,871, 4,710,134 and 4,941,832. The disadvantage of this type socket is shortage of reliably sufficient retention of the chip within the socket, so the chip carrier may be subject to removal from the socket if a vibration occurs thereof.
Compared with the first type socket, the second type socket furthermore has an additional cover latchably positioned on the socket base for assuring that the chip carrier seated within the socket is impossible to leave therefrom even in a worst vibration situation. This type socket can be represented by Plastic Quad Flat Pack (PQFP) socket, and includes U.S. Pat. Nos. 4,427,249, 4,583,806, 4,691,975, 4,746,299, 4,758,176, 4,846,704, 4,872,845, 4,886,470, 4,887,969, 4,968,259, 5,007,845 and 5,009,609. In this type socket, the common way to couple the leads of the chip carrier to the contacts of the socket is to use a projecting bar which integrally extends from the cover and directly presses the leads against the contacts or directly presses the contacts against the leads.
Regardless of either aforementioned type, the contact of the socket needs not only a desired resilient character which can provide a bumper function to reduce the influence due to vibration and can lower the insertion force of the chip carrier required when the chip carrier is loaded into the socket such as the PLCC socket, but also a sufficient retention strength which can efficiently make the contact tightly mechanically engage the lead of the chip carrier for reliable electrical connection. It can be seen that the contacts shown in the aforementioned U.S. Pat. Nos. 4,691,975, 4,846,704, 4,886,470 and 5,009,609, are at best shaped curved in order to achieve such functions. Unfortunately, generally in the socket there is no sufficient room for a contact design which has a compound curvilinear configuration. Also, an improper complex curved shape of the contact will make signal paths long to thereby increase inductive reactance and delay signal transmission.
For the PQFP socket, although the retention factor may not be so critical for the contact due to the fact that the additional cover will provide the necessary engagement force between the leads of the chip carrier and the contacts of the socket, and assure that the chip carrier can not leave from the socket, it is still required that the contact have a proper resilience which can absorb the amount of vibration energy occurring thereof for preventing the fragile leads from damage. This requirement results from the fact that in the current PQFP sockets, the projecting bar which presses against the contacts or the leads is integrally formed of the socket which is made of plastic, thus providing a poor bumping effect in relation to the contacts or the leads.
Another problem in some PQFP sockets is due to the additional cover. Generally speaking, the cover occupies a significantly great space relative to the socket base in a vertical direction. In contrast, with the introduction of notebook computers, the need has arisen to develop lower height sockets for chip carriers so the profile of the computer can be kept as slim as possible. However, the standard PQFP chip carrier has maximum allowable height of 4.57 mm, and the current PQFP socket, including the cover to sandwich the chip therebetween, has an overall height exceeding 9 mm. This height is too great for use in a notebook computer.
One reason for this thicker dimension of the socket is that it is uneasy to design a contact in a small space to connect with the chip lead from the bottom side of the socket while still keeping the contact owning the sufficient desired resilience and retention force. Another reason is related to the structure and the positioned of the cover in relation to the socket base, so the combination of the cover and the socket base where the chip carrier is positioned has such a considerable height.
Accordingly, to improve the first problem, an optional design is to load the contacts from the top so that it is easier to design a contact configuration providing a sufficiently proper downward retention force for engagement with the corresponding lead of the chip carrier. While, for a surface mount socket because the bottom portion of the contact is required to extend horizontally and mount on the board on which the socket is seated, it is almost impossible to have a unitary surface mount contact loaded into the socket from the top. Accordingly, using a two-piece contact, i.e. an upper contact loaded from the top and a lower contact loaded from the bottom, to replace the original one-piece contact for facilitating the better electrical connection among the lead of the chip carrier, the board, and the contact itself, is an object of the present invention.
Another object of the present invention is to provide a PQFP socket having upper contacts corresponding to respective leads of the chip carrier and lower contacts. The upper contacts replace the current projecting bars to press the leads of the chip carrier against the lower contacts for enhancement of contact force therebetween.
Still, an object of the present invention is to provide a PQFP socket in which the cover may not need additional securing means to latchably engaging the socket base. In other words, the combination via the upper contacts and the lower contacts provides the sufficient retention force to sandwich the chip carrier between the cover and the socket base without other additional latch means.
Yet, another object of the present invention is to provide a PQFP socket in which the chip carrier can be retained within the central opening of the cover first, and then the chip associated with the cover is positioned onto the socket. This feature is different from the current PQFP socket assembly where the chip carrier is required to be assembled to the socket base first, and then the cover is fastened to the socket base to retain the chip carrier therebetween. This unique feature makes assembly process flexible.
Further, another object of the present invention is to provide a PQFP socket which has restricting bars or retention sections to abut against the fixed ends of the leads of the chip carrier for prevent the chip carrier from upward movement.